Responsa for Nedarim 151:1-152:99:1

אימא סיפא אמרו לו לר"א אם מטבילין כלי טמא ליטהר יטבילו כלי לכשיטמא ליטהר שמע מינה חיילין

Then consider the second clause: They [the Rabbis] said to R. Eliezer: If an unclean utensil is immersed in order to purify it, shall a clean utensil be immersed, so that on [subsequently] becoming defiled it shall [simultaneously] become clean!<span class="x" onmousemove="('comment',' Surely not. ');">¹</span> This proves that they do take effect.<span class="x" onmousemove="('comment',' Since they compare it to the prior immersion of a utensil to render it clean after it has become defiled. ');">²</span>

Teshuvot Maharam